Pipe structure of branch pipe line

ABSTRACT

A water pipe structure of a branch pipe line ( 12 ) connected to a main pipe ( 22 ) for flowing water therethrough, wherein a cavity flow suppressing means ( 30 ) is installed between the main pipe ( 22 ) and the branch pipe line ( 12 ) or in the branch pipe line to suppress a cavity flow produced in the closed branch pipeline, whereby the adverse effect of a thermal stratification formed by the cavity flow on the pipe can be eliminated.

FIELD OF THE INVENTION

[0001] The invention relates to a water piping arrangement, and inparticular to a water piping arrangement for avoiding adverse effect tothe piping system based on a thermal stratification which is formed by acavity flow which is generated in a closed pipe branched from a mainpipe

DESCRIPTION OF THE PRIOR ART

[0002] A various branch pipes are generally connected to a main waterpipe in a power plant or the other types of plant, in which some branchpipes are used during only the starting operation or the maintenance ofthe plant and closed by a shut-off valve provided in the branch pipesafter the operation of the plant is transferred to a normal operation.

[0003] The inventors of the present application have found that such abranch pipe closed by a shut-off valve (hereinafter, referred to aclosed branch pipe) functions as a deep recess formed in a main pipe,and that, within the closed branch pipe, a cavity flow is induced by thewater flow in the main pipe. If the cavity flow is affected by a heatdissipating action of the wall of the branched pipe, a thermalstratification appears in the water within the closed branch pipe, andthe water temperature is suddenly changed across the thermalstratification so that a large thermal stress is generated in the pipe.

[0004] In the prior art, such a thermal stress in the pipes, based onthe thermal stratification generated by the cavity flow, is notconsidered in the calculation of the piping design. If a thermalstratification appears in a pipe, in particular in an elbow joint of thepiping system, a crack may be resulted in the elbow joint due to thethermal stress in the elbow joint.

DISCLOSURE OF THE INVENTION

[0005] The invention is directed to solve the above-described problem ofthe prior art, and to provide a piping arrangement of a branch pipe foravoiding adverse effect to the piping system, based on a thermalstratification generated by a cavity flow in the closed branch pipe.

[0006] According to the invention, there is provided a pipingarrangement which comprises a main pipe allowing a water flow; a branchpipe connected to the main pipe; and cavity flow inhibiting meansprovided between the main pipe and the branch pipe.

[0007] The cavity flow inhibiting means may comprise:

[0008] a swirl preventing plate including at least two plates whichintersect each other with an intersecting line extending in thedirection of the flow in the branch pipe;

[0009] a sleeve which has a inner diameter larger than the outerdiameter of the branch pipe and enclose a portion of the branch pipeconnected to the main pipe;

[0010] a deflecting member provided over a portion of the branch pipeconnected to the main pipe;

[0011] an orifice provided in the branch pipe;

[0012] a tube member which is provided in a portion of the branch pipeconnected to the main pipe and has different inner diameters one ofwhich is larger than that of the closed branch pipe;

[0013] an entrance radius enlarged portion with the sectional area ofits flow channel being gradually reduced from the main pipe toward thebranch pipe. cross pipe; or

[0014] an inclined connecting pipe for obliquely connecting the branchpipe to the main pipe.

[0015] According to another feature of the invention, a pipingarrangement of a branch pipe connected to a main pipe for allowing waterto flow therethrough is provided. The branch pipe includes a cross pipeconnected perpendicularly to the main pipe and a horizontal pipeconnected to the cross pipe by an elbow joint so as to horizontallyextend. The piping arrangement is characterized by the elbow joint beingdisposed in an area nearer than a transition ozone where a cellularvortex, generated in the branch pipe, is transformed into twistervortex.

[0016] The elbow joint is preferably disposed in a range within sixtimes of the inner diameter of the branch pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1A is a schematic illustration of a laboratory equipmentshowing the formation of a cavity flow generated in a closed branchpipe;

[0018]FIG. 1B is a section along line b-b in FIG. 1A;

[0019]FIG. 2 is the laboratory equipment showing from another direction;

[0020]FIG. 3 is a schematic illustration of the laboratory equipmentshowing its configuration;

[0021]FIG. 4A is a section of a piping arrangement similar to FIG. 1A,showing an example of the cavity flow inhibiting means;

[0022]FIG. 4B is a section along line VI-Vi in FIG. 4A,

[0023]FIG. 5 is a section of a piping arrangement similar to FIG. 4A,showing another embodiment of the cavity flow inhibiting means;

[0024]FIG. 6 is a section of a piping arrangement similar to FIG. 4A,showing another embodiment of the cavity flow inhibiting means;

[0025]FIG. 7A is a section of a piping arrangement similar to FIG. 4A,showing another embodiment of the cavity flow inhibiting means;

[0026]FIG. 7B is a perspective view showing an example of an orifice asthe cavity flow inhibiting means;

[0027]FIG. 7C is a perspective view showing another example of anorifice as the cavity flow inhibiting means;

[0028]FIG. 8 is a section of a piping arrangement similar to FIG. 4A,showing another embodiment of the cavity flow inhibiting means;

[0029]FIG. 9A is a section of a piping arrangement similar to FIG. 4A,showing an entrance radius enlarged portion as the cavity flowinhibiting means;

[0030]FIG. 9B is a section of a piping arrangement similar to FIG. 4A,showing another example of the entrance radius enlarged portion as thecavity flow inhibiting means;

[0031]FIG. 10 is a section of a piping arrangement similar to FIG. 4A,showing another embodiment of the cavity flow inhibiting means; and

[0032]FIG. 11 is a section of a piping arrangement illustrating a closedbranch pipe which is connected to a main pipe.

THE MOST PREFERRED EMBODIMENT

[0033] A various preferred embodiments of the invention will bedescribed below with reference to the accompanying drawings.

[0034] With reference to FIG. 11, a piping system to which the presentinvention is applied is shown. In FIG. 11, a branch pipe 10 is connectedto a main pipe 22, providing a water main line (hereinafter, referred tomain pipe 22), at a junction 22 a. The branch pipe 10 has a cross pipe12 and a horizontal pipe 14 connected to the cross pipe 12 by an elbowjoint 14, and forms a closed branch pipe when a valve 20, provided in anextended portion 18 of the horizontal pipe 14, is closed. The extendedportion 18 is not limited to the horizontal configuration.

[0035] In a large plant such as an electric power plant, a number ofbranch pipes are connected to a main pipe. Some of the branch pipes areused during only maintenance or the starting operation of the plant, andare not used during the normal operation of the plant with the valves onthe branch pipes closed after the plant is started. A high temperaturewater often flows through such a main pipe. For example, a hot water ofabout 200 Celsius degrees or higher flows through a boiler water supplypipe in a conventional electric power plant and a water of about 300Celsius degrees or higher flows through a primary cooling system in anuclear power plant.

[0036] The branch pipe 10 connected to the main pipe 22 becomes a closedbranch pipe when the valve 20 is closed. In the prior art, the flow insuch a closed branch pipe is not considered in the piping design. In theprior art, it has been assumed that because a thermal medium or thewater in the main pipe 22 does not flow into the closed branch pipe 10,the temperatures of the closed branch pipe 10 and the water thereingradually reduce and the temperature of the pipe reduces, in thedirection from the junction toward the distal end thereof, from thetemperature of the junction 22 a, connected to the main pipe 22, to theambient temperature due to the thermal dissipation through the wall ofthe closed branch pipe 10 after the start of the plant.

[0037] With reference to FIGS. 1-3, a laboratory equipment used forvisualization experiment of a cavity flow executed by the inventors willbe described below.

[0038] In FIG. 3, a visualization apparatus 100 includes a main pipe110, a branch pipe 120, a pump for driving a water of a predeterminedtemperature through the main pipe 112 and a tank 104 for holding thewater of the predetermined temperature. Within the tank 104, an electricheater 106 for maintaining the temperature of the water in the tank 104at the predetermined temperature with an electric power source 108supplying the electric power to the electric heater 106. The main pipeincludes an outlet pipe 110 a, a horizontal pipe 110 c providing anentrance region connected at the downstream of the outlet pipe 110 athrough an elbow joint 110 b, a T-joint 110 d for connecting a branchpipe 120 to the horizontal pipe 110 c and a return pipe 110 e forconnecting the T-joint 110 d and the tank 104. The branch pipe 120includes a vertical pipe 120 a connected to the T-joint 110 d and ahorizontal pipe 120 c connected to the lower end of the cross pipe 120 aby an elbow joint 120 b with the distal end of the horizontal pipe 120 cbeing closed by a blind cover 120 d.

[0039] In this connection, in the experiment, a pipe of nominal size of200A was used as the main pipe 110 and a pipe of nominal size of 100Awas used as the closed branch pipe 120. The horizontal pipe 110 c,providing the entrance region of the main pipe 110, has 10 m length toeliminate the influence of flow of the pump 102 and the elbow joint 110b. The length of the horizontal pipe 120 c of the closed branch pipe 120is 2300 mm. Further, the length of the cross pipe 120 a (including thevertically branched portion of the T-joint 110 d) is defined by L1-11.3d(d is the internal diameter of the horizontal pipe 110 c). On the otherhand, the flow within the return pipe 110 e has little influence to theflow in the closed branch pipe 120, and therefore, the arrangementthereof is not limited. A core 112 is disposed in the T-joint 110 d ofthe main pipe 110. By changing the size of the core 112, the flowvelocity of the water through the T-joint 110 d is changed.

[0040] Further, in the experiment, case (i), the normal temperatureswater, a water of around 20 Celsius degrees, was used for the flows inboth the water in the main pipe 110 and the closed branch pipe 120, andcase (ii), a hot water, heated to 60-70 Celsius degrees by the electricheater 106 in the tank 104, was used for the flow through the main pipe110 and the normal temperature water was used in the closed branch pipe120, were compared. In this connection, the flow in the closed branchpipe 120 was observed by using ink, bubble and polystyrene particleswith the relative density being previously adjusted. Further, in orderto observe the decay in the rotating velocity of the swelling flowgenerated in the closed branch pipe 120, a hot film was disposed at alocation from the wall surface to measure the velocity of the downflow.

[0041] The observation results of the flow, when the normal temperaturewater is used (case (i)), were shown in FIGS. 1 and 2. In the upper endregion in the closed branch pipe 120 or the region within the branchpipe 120 adjacent the junction to the main pipe 110, indicated byreference symbol “I” in FIGS. 1 and 2, a cellular vortex, vortexfluctuating strongly as a two-dimensional cavity flow, is induced by theflow through the main pipe 110. With reference, in particular, to FIG.1B, which is a section along line b-b in FIG. 1A, in the region I, apair of left and right, relative to the main stream in the main pipe110, vortexes are formed, which develop to the cellular vortex. Thecellular vortex has a shape in the form of a hair pin including adownflow in the downstream side region 122 relative to the flowdirection in the main pipe 110 and a upflow in the upstream side region124.

[0042] In region II under the region I, the cellular vortex becomeunstable and unclear, and develop to a twister vortex described below.Thus, it is assumed that the region II provides a transition zone fromthe region I where the clear cellular vortex appears and to the regionIII where the twister vortex appears. A twister vortex, which includes arotating downflow along the pipe wall and a upflow at the central regionof the pipe, appears in the region III under the region II. The reasonfor the development of the above-described cellular vortex to thetwister vortex is assumed that the cellular vortex cannot exist stablein the region II because of the circular cross-sectional shape of thebranch pipe 120.

[0043] As described above, clear cellular vortex is appears in theregion I. The cellular vortex has strong flow components in the axialdirection of the closed branch pipe 120. The cellular vortex in theregion II is unstable and unclear but also have strong flow componentsin the axial direction. In the experiment (i) shown in FIGS. 1 and 2, itwas observed that the twister vortex in the region III extends deeplyinto the horizontal pipe 120 c beyond the elbow joint 120 b. The twistervortex is a helical vortex having strong circumferential flow componentsand weak axial flow components. In the case of experiment (i), it wasobserved that the rotating flow disappears at the distal end of thetwister vortex and weakly layered natural circulation is generated. Thereason for the generation of the natural circulation is assumed that thetemperature of the water flowing through the main pipe 110 becomesslightly higher than the normal temperature of the water in the closedbranch pipe 120 due to the heat input from the pump 102.

[0044] On the other hand, in the case of the experiment (ii), when thetemperature difference between the waters in the main pipe 110 and inthe closed branch pipe 120 is about 40 Celsius degrees, a stable thermalstratification appears in the middle of the elbow joint 120 b and theabove-described natural circulation is inhibited. In particular, theboundary surface of the thermal stratification appears in the elbowjoint 120 b at level T1, substantially the same as the top of thehorizontal pipe 110 c. The reason for this is assumed that the axialflow components of the twister vortex is gradually decayed and the heatdissipation through the horizontal pipe 110 c increases toward thedistal end thereof so that the stability of the thermal stratificationis increased due to the turbulence inhibitory action by the buoyantforce.

[0045] Thus, it is assumed that the cavity flow induced within theclosed branch pipe 120 is extended to the end of the twister vortex, andthe length of the cavity flow in the closed branch pipe 120 is definedby the decay characteristics of the axial components of the twistervortex and the stability of the thermal stratification.

[0046] In this connection, the twister vortex did not appear within aregion from the opening 114 to the range of about six times of the innerdiameter “d” of the closed branch pipe 120. Thus, the terminal end ofthe transition zone II was observed at a position farther than the rangeof six times of the inner diameter “d” of the closed branch pipe 120.This is because the cellular vortex has an extent about three times ofthe inner diameter of a pipe and the region II is a zone where thesecond cellular vortex is generated.

[0047] When the thermal stratification is generated, in the proximalregion from the thermal stratification T1 to the opening 114, the waterin the main pipe 110 circulates in so that the temperature thereof issubstantially the same as that of the water in the main pipe 110. In thedistal region from the thermal stratification T1, the water in the mainpipe 110 does not circulate in so that the temperature is maintained tothe initial temperature, i.e., about 20 Celsius degrees so that thetemperature suddenly changes across the thermal stratification T1 and asteep temperature gradient is generated. This makes a large thermalstress in the pipe around the thermal stratification T1. As describedabove, when a large temperature difference between the water in the mainpipe 110 and that in the closed branch pipe 120 is generated, thethermal stratification T1 appears in the elbow joint 120 b at the levelthe same as the top of the horizontal pipe 110 c. The elbow joint 120 bis a member which has tendency to be broken when a thermal stress isapplied. Therefore, the condition in which a thermal stratificationappears in an elbow joint of a closed branch pipe in a plant for a longterm, it is expected that the elbow joint may be broken. Therefore, itis important to prevent the generation of a thermal stratification in anelbow joint due to the cavity flow induced in such a closed branch pipe.

[0048] A thermal stratification is not generated or is generated in theclosed branch pipe near the opening 114, if the water in the main pipeis prevented from circulating in the deep location in the closed branchpipe by preventing or inhibiting the generation of a cavity flow, sincethe thermal stratification is generated in the deep location in theclosed branch pipe, as described above, by the circulation of the waterin the main pipe into the closed branch pipe due to the cavity flow, asone of the measures for preventing the elbow joint 120 b from beingaffected by a large thermal stress.

[0049] With reference to FIGS. 4-10, various embodiments of cavity flowinhibiting means will be described below.

[0050] In the embodiment of FIG. 4, a swirl preventing plate 30 isprovided as the cavity flow inhibiting means. The swirl preventing plate30 comprises two plate members 30 a and 30 b which extend in planesincluding the axis of the cross pipe 12. The plate members 30 a and 30 bintersect, preferably perpendicularly at the center of the closed branchpipe 120 a as shown in FIG. 4E, to each other with an intersecting lineextending in the flow direction. The swirl preventing plate 30 ispreferably disposed in the transition region II or at a location in theregion III adjacent the transition region II where the twister vortexmay be generated. The swirl preventing plate 30 divides the inside ofthe cross pipe 12 into four volumes. Therefore, the generation of atwister vortex is prevented so that a cavity flow cannot flow beyond theswirl preventing plate 30.

[0051] In the embodiment of FIG. 5, a sleeve 32 with a bottom isprovided as the cavity flow inhibiting means. The sleeve 32 comprises aperipheral wall 32 a having an inner diameter larger than the outerdiameter of the cross pipe 12 and a bottom 32 b which is connectedbetween the peripheral wall 32 a and the cross pipe 12. The sleeve isdisposed so as to enclose the opening 22 a or the junction connected tothe main pipe 22, Thus, the provision of the sleeve 32 around theopening 22 a of the cross pipe 12 to the main pipe 22 allows the waterwithin a volume 32 between the cross pipe 120 a and the peripheral wall32 a to move by the shear action of the water flowing through the mainpipe 22 so that turbulence is generated around the opening 22 a. Thisweakens the cellular vortex generated in the cross pipe 12 to preventthe cavity flow from entering into the closed branch pipe 10.

[0052] In the embodiment of FIG. 6, a scoop or deflecting member 36 isprovided over the opening 22 a as the cavity flow inhibiting means. Thescoop or the deflecting member 36 has preferably a shape of a portion ofa sphere and prevents the cavity flow from entering into the closedbranch pipe 10 by reducing the shearing action of the water flowingthrough the main pipe 22 for the water in the closed branch pipe 10.

[0053] In the embodiment of FIG. 7A, an orifice 38 is provided as thecavity flow inhibiting means. The orifice 38 reduces the entrance of thecellular vortex and the twister vortex. Thus, the orifice 38 inhibitsthe formation of the cellular vortex in the form of a hear pin. Theorifice 38 may be formed by a central opening 38 b defined by an annularplate member 38 a, as shown in FIG. 7B. The orifice may include a risingportion or collar 38 e provided along the periphery of the centralopening 38 d of the annular plate member 38 c.

[0054] In the embodiment of FIG. 8, a tube member 40 having differentdiameters is provided as the cavity flow inhibiting means. The tubemember includes a peripheral wall 40 a, which has an inner diameterlarger than the inner diameter of the closed branch pipe 10 and isconnected to the main pipe 22, and an annular bottom portion 40 b whichis connected to the peripheral wall 40 a and the cross pipe 12. Thus,provision of the tube member 40 allows the flow, in the form of a hearpin induced by the flow through the main pipe 22, to impinge against thebottom portion 40 b of the tube member 40 and to be broken. Therefore,it cannot enter the cross pipe 12 of the closed branch pipe 10 so thatthe cellular vortex is weakened.

[0055] In the embodiment shown in FIG. 9, entrance radius enlargedportions 42 and 44 are provided as the cavity flow inhibiting means. Theentrance radius enlarged portions 42 and 44 are tubular members with thesectional area of their flow channels being gradually reduced from themain pipe 22 toward the cross pipe 12. In the embodiment of FIG. 9A, inparticular, the enlarged radius entrance portion 42 has a symmetricconfiguration relative to the axis of the cross pipe 12. In theembodiment of FIG. 9B, the enlarged radius entrance portion 44 has anasymmetric configuration, in which the upstream side in the flowdirection in the main pipe 22 relative to the axis of the cross pipe 12,perpendicularly intersects the main pipe 22 but on the downstream sidethe sectional area of the flow channel is gradually reduced. Thus, theprovision of the entrance radius enlarged portion 42 and 44 between themain pipe 22 and the cross pipe 12 weakens the cellular vortex,therefore, the twister vortex and the cavity flow in the cross pipe 12are weakened.

[0056] In the embodiment shown in FIG. 10, the closed branch pipe 10includes an inclined connecting pipe 46, as the cavity flow inhibitingmeans, for obliquely connecting the cross pipe 12 to the main pipe 22.In the embodiment shown in FIG. 10, the cross pipe 12 is connected tothe inclined connecting pipe 46 by a bend joint having 45 degrees andthe inclined connecting pipe 46 is connected to the main pipe 22 towardthe downstream of the flow in the main pipe 22 with angle of 45 degrees.Thus, provision of the inclined connecting pipe 46 between the crosspipe 12 and the main pipe 22 weakens the cellular vortex, therefore, thetwister vortex and the cavity flow in the cross pipe 12 are weakened.

[0057] In the embodiment described above, the generation of the cavityflow is prevented or inhibited to prevent the water in the main pipefrom circulating deeply into the closed branch pipe whereby theformation of the thermal stratification in the closed branch pipe 10 oradjacent the opening 22 a is prevented so as to prevent a large thermalstress in the elbow joint 14. in other word, in the embodiment of FIG.4-10, the formation of the thermal stratification is prevented or thethermal stratification is formed upstream of the elbow joint 14 on theother hand, if the thermal stratification is formed in the closed branchpipe 10 downstream of the elbow joint 14, the large thermal stress inthe elbow joint 14 can be prevented.

[0058] As described above, the cellular vortex has strong flowcomponents in the axial direction, which provide the water in the mainpipe 22 with driving force for circulation into the closed branch pipe10. If the axial flow components are large in the closed branch pipe 10,the stratification by buoyant force is avoided or reduced. Therefore,the thermal stratification does not appear in an area where the cellularvortexes exist. On the other hand, the axial flow components of thetwister vortex are weak so that a thermal stratification is easilyappears. Further, in the closed branch pipe 10, region I where a clearcellular vortex appears and region II where a unclear cellular vortexappears, the region II providing a transition zone from the cellularvortexes to the twister vortex are formed. Therefore, by disposing theelbow joint 14 between the regions I and II, the formation of thethermal stratification in the elbow joint 14 can be prevented. Inparticular, a cellular vortex has a size of three times of the innerdiameter of a pipe. Therefore, by disposing the elbow joint 14 within arange within six times of the inner diameter from the opening 22 a, theformation of the thermal stratification in the elbow joint can beprevented.

[0059] Incidentally, although the cross pipe 12 is shown to verticallyextend in the above-described embodiments, the invention is not limitedto this configuration and the cross pipe 12 may be extend vertically,horizontally or an angle therebetween.

[0060] Further, although it is described that the main pipe provides awater main line in a plant in the above-described embodiment, theinvention is not limited to this configuration and any water pipethrough which a hot water above 40 Celsius degrees flow at a relativelyhigh flow rate.

1. A piping arrangement, comprising: a main pipe which allows a waterflow; a branch pipe connected to the main pipe; and cavity flowinhibiting means provided between the main pipe and the branch pipe. 2.(Amended) A piping arrangement according to claim 1, wherein the cavityflow inhibiting means comprises a swirl preventing plate including atleast two plates which intersect each other with an intersecting lineextending in the direction of the flow in the branch pipe, the swirlpreventing plate being disposed in a transition region (II) or twistervortex region (III).
 3. (Deleted)
 4. A piping arrangement according toclaim 1, wherein the cavity flow inhibiting means comprises a deflectingmember provided over a portion of the branch pipe connected to the mainpipe.
 5. A piping arrangement according to claim 1, wherein the cavityflow inhibiting means comprises an orifice provided in the branch pipe.6. A piping arrangement according to claim 5, wherein the orificeincluding a central opening defined in an annular plate member, and acollar 38 e provided along the periphery of the central opening of theannular plate member.
 7. (Deleted)
 8. A piping arrangement according toclaim 1, wherein the cavity flow inhibiting means comprises an entranceradius enlarged portion with the sectional area of its flow channelbeing gradually reduced from the main pipe toward the branch pipe. crosspipe
 12. 9. A piping arrangement according to claim 8, wherein theentrance enlarged portion having an asymmetric configuration in whichthe upstream side, in the flow direction in the main pipe, relative tothe axis of the cross pipe, perpendicularly intersects the main pipe butat the downstream side, the sectional area of the flow channel isgradually reduced.
 10. A piping arrangement according to claim 1,wherein the cavity flow inhibiting means comprises an inclinedconnecting pipe for obliquely connecting the branch pipe to the mainpipe.
 11. A piping arrangement of a branch pipe connected to a main pipefor allowing water to flow therethrough, comprising: the branch pipeincluding a cross pipe connected perpendicularly to the main pipe and ahorizontal pipe connected to the cross pipe by an elbow joint so as tohorizontally extend; characterized in that the elbow joint is disposedin an area nearer than a transition zone where a cellular vortex,generated in the branch pipe, is transformed into twister vortex.
 12. Apiping arrangement according to claim 12, wherein the elbow joint isdisposed in a range within six times of the inner diameter of the branchpipe.